Handle rate limiting

[jantman]

We've been having some AWS Limit Checker runs fail with 400 Errors like:

boto.exception.BotoServerError: BotoServerError: 400 Bad Request
<ErrorResponse xmlns="http://elasticloadbalancing.amazonaws.com/doc/2012-06-01/">
  <Error>
    <Type>Sender</Type>
    <Code>Throttling</Code>
    <Message>Rate exceeded</Message>
  </Error>
  <RequestId>fab58587-8841-11e5-938b-a9029686dd0d</RequestId>
</ErrorResponse>

Handle this by wrapping all AWS API queries in an exponential backoff (ideally one which saves state between runs).